Heat exchanger

ABSTRACT

A heat exchanger is disclosed, which comprises a plurality of heat-transfer elements (1) placed side by side each of which has more than one through-hole (3) and which are cyclically bent in a generally trapezoidal wave form in the direction of the flow of a fluid, the bends in one heat-transfer element (1 ) being in phase with those in an adjacent heat-transfer element (1) in such a manner that the main stream of said fluid will flow not through the holes in each of said heat-transfer elements (1) but through the passage formed by adjacent heat-transfer elements (1). This arrangement not only provides improved heat-transfer characteristics; it also serves to offer a lighter product because of the presence of throught-holes (3).

TECHNICAL FIELD

The present invention relates to a heat exchanger, in particular to animprovement of the heat-transfer characteristic of a heat-transferelement such as a heat-transfer fin.

BACKGROUND ART

An example of the heat-transfer unit used in a prior art heat exchangeris shown in FIG. 12.

The drawing is a partial perspective view of the conventionalheat-transfer unit that is generally indicated by (1) and disposed inthe direction of the flow of a fluid (A) (as indicated by the arrows).The heat-transfer element (1) is basically composed of heat-transferfins, a heat generator, a heat absorber, a heat accumulator, and a heatradiator. In FIG. 6, the heat-transfer unit consists of a plurality ofheat-transfer elements (1a), (1b) and (1c) that are stacked one on topof another and the fluid flows through the passage formed by adjacentheat-transfer elements. Each heat-transfer element (1) is cyclicallybent in the direction of fluid flow in the form of trapezoidal waves,the bends in one element being in phase with those in an adjacentelement.

The heat-transfer unit of the type described above is hereinafterreferred to as an imperforate trapezoidally corrugated plate.

FIG. 13 is a partial perspective view of another conventionalheat-transfer unit that consists of a plurality of heat-transferelements (1) in a plane plate form that are disposed in the direction ofthe flow of a fluid (A) (as indicated by the arrows). This type ofheat-transfer unit is hereinafter referred to as parallel plates.

FIG. 2 is a graph showing the heat-transfer characteristics of the twoconventional types of heat-transfer unit, in which the characteristicsof the imperforate trapezoidally corrugated plate are indicated by ○ andthose of the parallel plates by . The symbols on the x- and y-axes ofthe graph are:

    Re=v·De/ν: Reynolds number;

    Nu=h/De/λ: Nusselt number

where

v: maximum velocity of wind passing through the heat-transfer unit;

De: spacing between heat-transfer surfaces multiplified by a factor of2;

h: heat transfer rate;

ν: fluid dynamic viscosity coefficient; and

λ: fluid heat conductivity.

As is clear from FIG. 2, the imperforate trapezoidally corrugate platetype heat-transfer unit shown in FIG. 12 and the parallel-plate typeheat-transfer unit shown in FIG. 13 have essentially the sameheat-transfer characteristics. In the heat-transfer unit of the typeshown in FIG. 12, the fluid flows along the individual heat-transferelements and this would provide the unit with heat-transfercharacteristics which are essentially the same as those exhibited by theparallel-plate type heat-transfer unit.

DISCLOSURE OF THE INVENTION

The heat exchanger of the present invention comprises plurality ofheat-transfer elements placed side by side each of which has more thanone through-hole and which are cyclically bent in a generallytrapezoidal waveform in the direction of the flow of a fluid, the bendsin one heat-transfer element being in phase with those in an adjacentheat-transfer element in such a manner that the main stream of saidfluid will flow not through the holes in each of said heat-transferelements but through the passage formed by adjacent heat-transferelements. Because of this arrangement, the fluid flowing along onesurface of each heat-transfer element will be sucked in through theholes and blown out of them to flow along the other surface of theheat-transfer element. In the portion where the fluid is sucked in, thethickness of a temperature boundary layer is reduced and in the portionwhere the fluid is blown out, replacement of fluid bodies will occur,thereby promoting heat-transfer so as to provide improved heat-transfercharacteristics for the heat-transfer elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view showing a heat-transfer unitaccording to a first embodiment of the present invention; FIG. 2 is agraph showing the heat-transfer characteristics of the heat-transferunit according to the first embodiment of the present invention, as wellas two prior art heat-transfer units; FIG. 3 is an illustration of theprofile of pressures on the wall surface of a bent fluid passage as afunction of the direction of fluid flow; FIGS. 4 and 5 are a partialcutaway view and a partial cross-sectional view of heat-transfer unitsaccording to a second and a third embodiment, respectively, of thepresent invention; FIG. 6 is a partial cross-sectional view ofheat-transfer units according to a fourth, a fifth and a sixthembodiment of the present invention; FIG. 7 is a characteristic diagramshowing the relative promotion of heat-transfer as achieved in thefourth embodiment of the present invention; FIG. 8 is a characteristicdiagram showing the relationship between the diameter of through-holesin the heat-transfer unit of the fifth embodiment and the relativepromotion of heat-transfer achieved; FIG. 9 is a characteristic diagramshowing the relationship between the amount of opening in theheat-transfer unit of the sixth embodiment and the relative promotion ofheat-transfer achieved; FIG. 10 is a characteristic diagram showing therelationship between the angle of inclination of oblique surfaces in theheat-transfer unit of the seventh embodiment and the ratio ofoutside-tube heat-transfer coefficient to wind pressure loss; FIG. 11 isa perspective view of the essential parts of a heat-transfer unitaccording to an eighth embodiment of the present invention; and FIGS. 12and 13 are partial perspective views of two different prior artheat-transfer units.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 1 is a partial perspective view of a heat-transfer unit accordingto a first embodiment of the present invention. The heat-transfer unitof this embodiment differs from the one shown in FIG. 12 in that aplurality of through-holes (3) are made in individual heat-transferelements.

The heat-transfer characteristics of this heat-transfer unit (1)(hereinafter referred to as a perforated trapezoidally corrugated plate)are shown in FIG. 2 in terms of experimental values by Δ. It can be seenthat this heat-transfer unit has improved heat-transfer characteristicsover the imperforate trapezoidally corrugated plate shown in FIG. 12.

The reason for this effect would be as follows.

FIG. 3 is an illustration showing how the pressure on the wall surfaceof a common bent fluid passage will vary in the direction of fluid flow(also see Izumi et al., "Fluid Motion and Heat Transfer in a CorrugatedChannel", in Transactions of the Japan Society of Mechanical Engineers,vol. 46, No. 412). FIG. 3(a) shows a cross section of the corrugatedchannel, in which (10a) and (10b) are each a bent wall.

FIG. 3(b) shows the distribution of dimensionless pressure on thesurface of each wall in the direction of fluid flow. At the sameposition in the direction of fluid flow, the pressure on wall (l0a) ishigh when the pressure on wall (l0b) is low, thereby creating a pressureprofile for the two walls that varies in opposite directions. Therefore,if fluid channels of the configuration shown in FIG. 3(a) are arrangedone on top of another, a pressure difference is produced between the twodies (the obverse and reverse side) of each wall of the corrugatedchannel and as shown in FIG. 3(b), this pressure difference iscyclically inverted in the direction of fluid flow.

Because of this mechanism, in the heat-transfer unit (1) shown in FIG.1, a pressure difference is produced between the two sides (the obverseand reverse sides) of each wall of the corrugated channel at every bentportion and part of the fluid will flow across the wall through holes(3). Therefore, if a heat-transfer unit is constructed in the way shownin FIG. 1, the fluid flowing along one surface of each heat-transferelement will be sucked in through-holes (3) and blown out of them toflow along the other surface of the heat-transfer element, with thesurface where the fluid is sucked in alternating with the surface wherethe fluid is blown out in the direction of fluid flow. In the surfacewhere the fluid is sucked in, the thickness of a temperature boundarylayer is sufficiently decreased to achieve significant enhancement ofheat-transfer. In the surface where the fluid is blown out, replacementof fluid bodies takes place, which also leads to an improved performanceof heat-transfer. These two effects would combine to accomplish dramaticpromotion of heat-transfer.

In addition, the heat-transfer unit of the first embodiment of thepresent invention is so designed that the main stream of the fluid (A)will chiefly flow along the individual heat-transfer elements (1) withonly a small amount of the fluid flowing through the holes (3) as abranch stream.

In other words, in one cycle of bends in each heat-transfer element (1),the greater part of the fluid will flow through the same passage on onesurface of the element and only a limited portion of the fluid will flowacross the element through-holes (3). As a result, the main stream ofthe fluid will flow undeflected along the individual heat-transferelements (1).

Second Embodiment

FIG. 4 is a partial cutaway view of a heat exchanger according to asecond embodiment of the present invention which is a corrugated fintype heat exchanger commonly used as a radiator in such applications asautomobiles.

In FIG. 4, (1) is a first heat-transfer unit of the same type as used inthe first embodiment which consists of a plurality of heat-transferelements each of which has more than one through-hole (3) and which arecyclically bent in a generally trapezoidal waveform in the direction ofthe flow of a secondary fluid (A) such as air, the bends in oneheat-transfer element being in phase with those in an adjacentheat-transfer element, and (2) is a second heat-transfer unit that has atemperature difference form the first heat-transfer unit (1) and whichis in the form of a water pipe through which a primary fluid (B) such asengine cooling water flows. The water pipe (2) is positioned normal tothe direction of the flow of the secondary fluid (A). The firstheat-transfer unit (1) is thermally coupled to the second heat-transferunit (2) so that heat exchange will take place between the primary fluid(B) and the secondary fluid (A).

Third Embodiment

FIG. 5 is a partial cross-sectional view of a heat exchanger accordingto a third embodiment of the present invention which is a plate fin typeheat exchanger for use in air-conditioning. In FIG. 5, a pipe serving asa second heat-transfer unit (2) passes through a first heat-transferunit (1) of the same type as used in the second embodiment and ispositioned normal to the direction of flow of fluid (A).

In the heat exchangers of the types shown in FIGS. 4 and 5, the secondheat-transfer unit (2) through which the primary fluid (B) flowsgenerally has good heat-exchanging characteristics because water istypically used as the primary fluid (B), and it is the heat-transferfins, or the first heat-transfer unit (1) through which the secondaryfluid (A) such as air flows; the are desired to be improved in terms ofheat-transfer characteristics. Heat exchangers having improvedperformance in this respect can be attained by providing through-holes(3) in the same way as described in connection with the previousembodiments of the present invention.

Fourth Embodiment

A fourth embodiment of the present invention is hereinafter describedwith reference to FIG. 6. In this embodiment, the heat-transfer unit (1)is designed to meet certain dimensional specifications.

FIG. 6 is an enlarged cross section of FIG. 1 and the components whichare the same as those shown in FIG. 1 are identified by like numerals.

In FIG. 6, l signifies the projected length of the heat-transfer surfaceof a heat-transfer element (1) which is in the area corresponding to onehalf cycle of a series of generally trapezoidal bends formed in thedirection of fluid flow, the projection being made normal to thedirection of fluid path, and L denotes the overall length of theheat-transfer unit.

First, the periodicity of trapezoidal forms is explained. the method ofthe present invention for achieving accelerated heat-transfer is chieflybased on the heat-transfer promoting effect of uniform sucking andblowing of a fluid but at the same time, the effect of repeated approachzones due to the cyclic changes of a temperature boundary layer thatresult from the fluid coming into and out of the heat-transfer unitwould also be significant. In other words, length l rather than theperiodicity of trapezoidal forms would cause a predominant effect. Basedon this understanding, the present inventors formulated the results oftheir heat-transfer experiments in terms of l/L, the ratio of length lto the length L of heat-transfer unit (1).

The results of an experiment conducted in air to investigate therelationship between the value of l/L and the relative promotion of heattransfer are shown in the characteristic diagram of FIG. 7, in which they-axis represents the relative promotion of heat transfer and the x-axisthe value of l/L, with the Reynolds number Re being taken as aparameter.

In FIG. 7, Re (basically representing the magnitude of fluid velocity)is given by: ##EQU1##

The relative promotion of heat-transfer, taken against the case ofparallel plates in which the heat-transfer unit consists of a pluralityof parallel plane plates are arranged together, is given by: ##EQU2##The average Nusselt number Nu is a dimensionless number that representsheat-transfer rate and is given by: ##EQU3##

As is clear from FIG. 7, the profile of the relative promotion of heattransfer vs l/L is curved upward and in the range of l/L 0.25, theheat-transfer rate of the system of the present invention is at least1.5 times as high as the value for the parallel plates. Thischaracteristic is substantially independent of the Reynolds number Re,as well as of other shape parameters although not shown in FIG. 7.Therefore, for the purposes of the present invention, l/L is suitably at0.25 and below.

The following are the dimensional ranges desired for other shapeparameters.

(a) diameter of through-holes (3): 0.5-6 mm

(b) relative opening of through-hole (3) (area of through-holes relativeto the area of an individual heat-transfer element: 0.05-0.40

(c) average distance between heat-transfer elements (1):

1-2 mm (for small-size unit such as one used for residentialair-conditioning)

6-10 mm (for medium-size unit).

The reason for the conclusion stated above with reference to FIG. 7would be that through-holes (3) which provide passages for fluid flowacross a heat-transfer element also serve as restarting points for thedevelopment of a temperature boundary layer (a so-called repetitioneffect of approach zones). As a result, the shorter the length (l) ofthe area where such through-holes exist, the greater the effect for thepromotion of heat-transfer.

However, if the value of l is too small, the heat-transfercharacteristics of the system under discussion will approach those ofparallel plates and the relative promotion of heat-transfer that isachieved is decreased rather than increased. In addition, for practicalreasons of machining, approximately 3 mm is the lower limit of l.

For attaining an effective and desirable relative promotion of heattransfer l/L is advantageously 0.3 and below, with l desirably rangingfrom 3 mm up to about 50 mm in practical situations.

Fifth Embodiment

A fifth embodiment of the present invention is hereunder described withreference to FIG. 6. In this embodiment, the size (diameter), d, of eachof the through-holes (3) in an individual heat-transfer element (1) isspecified to be within a certain range. If the relative opening ofthrough-holes (3), or the proportion of the heat-transfer element (1)taken by the opening of the holes, is written as β, and the widths ofadjacent fluid paths as A₁ and A₂ (in the case shown, A₁ =A₂), A₁ (A₂)=6mm, l=15 mm, L=100 mm and β=12.5% in the embodiment under discussion.

The method of the present invention for achieving acceleratedheat-transfer is largely based on the heat-transfer promoting effect ofa static pressure difference that is created between adjacent fluidpaths to have part of the fluid flow across a heat-transfer elementthrough-holes (3), and the size, d, of each through-hole (3) would havea strong effect on the characteristics of heat-transfer promotion.

Therefore, the present inventors investigated the relationship betweenthe value of hole diameter, d, and the relative promotion ofheat-transfer by experimentation in air. The results of the experimentare shown in FIG. 8.

In FIG. 8, the parameter Re is given by: ##EQU4## The y-axis in FIG. 8,represents the relative promotion of heat transfer, which is defined as:##EQU5##

The average Nusselt number Nu is a dimensionless number that representsheat-transfer rate and is given by: ##EQU6##

The characteristic shown in FIG. 8 is substantially independent of Re(basically representing the magnitude of fluid velocity), as well as ofother shape parameters although not shown in FIG. 8. According to theexperiment conducted by the present inventors, characteristics similarto that shown in FIG. 8 were obtained when the relative opening ofthrough-holes (3) was in the range of 0.05-0.4 and l/L being 0.25 orbelow.

According to FIG. 8, the profile of the relative promotion ofheat-transfer vs hole diameter, d, is curved upward and in the range ofd=0.5-4.5, the heat-transfer rate of the system of the present inventionis at least 1.5 times as high as the value for the parallel plates.

This would be explained as follows: even if the relative opening β isconstant, each heat-transfer element (1) has a finite plate thickness,and as the hole diameter, d, decreases, the resistance of through-holes(3) to fluid flow increases to such an extent that given a constantstatic pressure difference between adjacent fluid paths, a smalleramount of fluid will flow through holes (3) to cause a correspondingdecrease in the relative promotion, d, increases to a certain degree,the resistance to fluid flow of through-holes (3) having a constantvalue of β will remain constant, but if the value of, d, increasesprogressively, the pitch or the spacing of adjacent through-holes (3)also increases and the mechanism of heat-transfer promotion described inconnection with the first embodiment can no longer be maintained with asubsequent drop in the relative promotion of heat-transfer. For thesetwo reasons, there would be an appropriate value for the diameter, d, ofan individual through-hole.

In other words, it can be seen that for effectively increasing therelative promotion of heat-transfer, d is desirably within the range of0.5-4.5 mm.

Even in the case of through-holes which are not circular in crosssection, it goes without saying that comparable results will be attainedif the area of such non-circular holes is within the range of areas thathave equivalent diameters within the above-specified range.

Sixth Embodiment

A sixth embodiment of the present invention is hereunder described withreference to FIG. 6. This embodiment is characterized in that therelative opening of through-holes, β, is specified to be within acertain range. In this embodiment, the bends in one heat-transferelement are made in phase with those in an adjacent heat-transferelement, so that the distance, A₁ or A₂, between adjacent heat-transferelements (1) is generally constant, with A₁ being equal to A₂.

As already mentioned, the method of the present invention for achievingaccelerated heat transfer is largely based on the heat-transferpromoting effect of a static pressure difference that is created betweenadjacent fluid paths to have part of the fluid flow across aheat-transfer element through-holes (3) and, in this sense, the relativeopening β of through-holes (3) is a factor that directly governs thevolume of fluid flow. Therefore, it is assumed that β will have a verygreat effect on heat-transfer characteristics.

The results of an experiment conducted in air to investigate therelationship between the value of β and the relative promotion ofheat-transfer are shown in FIG. 9.

In FIG. 9, the parameter Re is given by: ##EQU7## and the results forRe=400, 750 and 2,000 are depicted. The y-axis in FIG. 9 represents therelative promotion of heat-transfer with the loss of heat-transfer areadue to through-holes being taken into account and is given by: ##EQU8##The average Nusselt number Nu is a dimensionless number that representsheat-transfer rate and is given by: ##EQU9## The characteristics shownin FIG. 9 is substantially independent of Re (basically representing themagnitude of fluid velocity), as well as of other shape parametersalthough not shown in FIG. 9.

According to FIG. 9, the profile of the relative promotion ofheat-transfer vs relative opening β is curved upward and in the vicinityof β=0.05-0.5, the heat-transfer rate of the system of the presentinvention is approximately twice the value for the parallel plates.

If evaluated without taking into account the loss of heat-transfer areadue to the presence of through-holes (3), the relative promotion of heattransfer increases gradually as the relative opening, β, and hence thevolume of fluid flow through-holes (3), increases.

However, the increase in relative opening β results in the decrease inheat-transfer area and evaluation of relative promotion of heat-transfertaking this loss of heat-transfer area into account provides the resultshown in FIG. 9.

The profile of relative promotion of heat-transfer shown in FIG. 9 isthe one which is observed in practical operations, so it can be seenthat for achieving effective relative promotion of heat-transfer, therelative opening β is desirably within the range of 0.05-0.5.

Needless to say, completely the same results will be attained even ifthe through-holes (3) have non-circular cross-sectional forms such asrectangles.

The following are the dimensional ranges desired for other shapeparameters.

(a) diameter, d, of through-hole (3): 0.6-6 mm

(b) l/L: no more than 0.3 (l≧2.5 mm)

(c) average distance between adjacent heat-transfer elements (1):

1-2 mm (for small-size unit such as one used for residentialair-conditioning)

6-10 mm (for medium-size unit).

Seventh Embodiment

In this embodiment, each of the trapezoidal bends in a heat-transferelement (1) is so designed that the inclined surfaces thereof will makean angle (θ) of 25-65° with respect to the direction of fluid flow asshown in FIG. 6. It has been found that if this design is adopted, α/ΔP,or the ratio of outside-tube heat-transfer rate to wind pressure loss,which is one of the important factors for the maintenance of theperformance of a heat exchanger becomes the highest for the same windvelocity as shown in FIG. 10.

This would be explained as follows: if the angle θ is too small, thedimension E of a trapezoidal bend taken in the direction of its heightbecomes smaller than the thickness of a temperature boundary layerformed in the direction of incidence of an air stream, with thesubsequent decrease in heat-transfer characteristics; if the angle θ isexcessive, the heat-transfer performance will not be greatly improvedand instead the wind pressure loss will increase to cause a drop in thecharacteristics of the system as a heat exchanger. Another problemassociated with the excessive value of θ is that it impairs structuralintegrity by increasing the chance of the formation of defective finsduring their molding.

Eighth Embodiment

In this embodiment, some of the through-holes (3) are so positioned thatthey extend across an inclined portion (4) of a heat-transfer element(1) to bridge adjacent flat portions (5).

The through-holes (3) formed in inclined portions (4) of a heat-transferelement (1) chiefly govern the loss of fluid flow whereas thethrough-holes (3) in flat portions (5) serve to improve heat-transferperformance. Therefore, if through-holes (3) are made at the positiondefined in the preceding paragraph, there will be no substantial changein heat-transfer performance for the same value of relative opening βand instead, the wind pressure loss will be decreased to achieve aconsequential improvement in α/ΔP, or the ratio of the outside-tubeheat-transfer rate to wind pressure loss. The reason for this decreasein the loss of fluid flow is that air flows into an enlarged portion ofa heat-transfer element on the downstream side through such holes (3) soas to decrease the fluid velocity in a reduced portion.

In the fourth to eighth embodiments described above, the values of l/L,l, d, β, θ, and the position of through-holes (3) in an inclinedportion, respectively, are specified as modifications of the firstembodiment, and it should be understood that similar modifications canbe made to each of the second and third embodiments by incorporatingnumerical limitations based on the same concept.

Advantages of the Invention

As described in the foregoing, according to the present invention, aplurality of heat-transfer elements each having more than onethrough-hole and which are cyclically bent in a generally trapezoidalwaveform in the direction of the flow of a fluid are placed side by sidein such a manner that the bends in one heat-transfer element will be inphase with those in a adjacent heat-transfer element and that the mainstream of said fluid will flow not through the holes in each of saidheat-transfer elements but through the passage formed by adjacentheat-transfer elements. This arrangement not only provides improvedheat-transfer characteristics; it also serves to offer a lighter productbecause of the presence of through-holes.

What is claimed:
 1. A heat exchanger comprising a plurality ofheat-transfer elements placed side by side each of which has more thanone through-hole and which are cyclically bent in a generallytrapezoidal waveform in the direction of the flow of a fluid, the bendsin one heat-transfer element being in phase with those in an adjacentheat-transfer element in such a manner that the main stream of saidfluid will flow not through the holes in each of said heat-transferelements but through the passage formed by adjacent heat-transferelements.
 2. A heat exchanger according to claim 1 wherein l/L is set toa value of no more than 0.3, l being the projected length of aheat-transfer element in the area corresponding to one half cycle of aseries of trapezoidal bends, the projection being made normal to thedirection of fluid path, and L being the length of each heat-transferelement.
 3. A heat exchanger according to claim 2 wherein l is at least2.5 mm.
 4. A heat exchanger according to claim 1 wherein the diameter,d, of each of the through-holes is within the range of 0.5-4.5 mm.
 5. Aheat exchanger according to claim 1 wherein the relative opening, β, ofthrough-holes is within the range of 0.05-0.5.
 6. A heat exchangeraccording to claim 1 wherein each of the trapezoidal bends in anindividual heat-transfer element is such that the inclined surfacesthereof make an angle, θ, of 25°-65° with respect to the direction offluid flow.
 7. A heat exchanger according to claim 1 wherein some of thethrough-holes are so positioned that they extend across an inclinedportion of a heat-transfer element to bridge adjacent flat portions. 8.A heat exchanger according to any one of claims 1 to 7 wherein each ofsaid heat-transfer elements is thermally coupled to a secondheat-transfer element having a temperature difference from said firstheat-transfer elements.
 9. A heat exchanger according to claim 8 whereinsaid second heat-transfer element passes through the stack of said firstheat-transfer elements and is positioned normal to the direction of theflow of the fluid flowing along said first heat-transfer elements.
 10. Aheat exchanger according to claim 8 wherein said second heat-transferelement is a pipe through which a second fluid flows.
 11. A heatexchanger according to claim 9, wherein said second heat-transferelement is a pipe through which a second fluid flows.